In an air conditioning system which performs ventilation or cooling and heating, especially in an air conditioning system which supplies air from one blower unit to one or a plurality of rooms via duct(s), a condition (hereinafter will be referred to as “pressure loss condition”) which determines a pressure loss in the air conditioning system, is different, depending on the installed state or use state of an air conditioning apparatus including the blower unit. For example, the pressure loss condition is different depending on a difference in the shape or length of the duct(s) connected to the air conditioning apparatus. Also, due to clogging of a filter attached to the outlet port or suction port of the air conditioning apparatus, the pressure loss condition changes with a passage of time. Due to the difference or change in the pressure loss condition with a passage of time, a static pressure required to obtain a specified air flow also changes. For this reason, even when a plurality of air conditioning apparatus of the same specification are driven at equal torque or an equal rotational speed, the resulting air flows (air volumes) may be sometimes non-uniform. Or, even when a particular air conditioning apparatus is driven at constant torque or a constant rotational speed, its air flow cannot be maintained at a constant value. Under the circumstances, in recent years, it is required that the air flow be controlled to be maintained at a target air flow, when the pressure loss condition or the static pressure changes.
As a technique for realizing the constant air flow control, for example, there is a method which uses a pressure sensor. However, this method has a problem that the pressure sensor is not easily installed or a reliability is low in long-time use. To solve this, study has been conducted for various methods which utilize a characteristic (hereinafter will be referred to as “blower characteristic”) unique to the blower unit, which is necessary for the constant air flow control without using the pressure sensor. In this method, prior to operation (running) under the constant air flow control, a measurement experiment which measures the relation between the physical amounts (motor speed, motor torque, etc.) of a motor of the blower unit, and the air flow, is conducted preliminarily. By utilizing the blower characteristics obtained in this measurement experiment, the constant air flow control is performed.
As such a technique of the constant air flow control, for example, Patent Literature 1 is known. According to the prior art disclosed in Patent Literature 1, the command speed of the motor which drives the blower unit is calculated according to the following formula (1):S*=S+K×(S−Sa)  (1)
S* indicates the command speed of the motor, S indicates a motor speed, Sa indicates a target motor speed, and K indicates a gain. The target motor speed Sa is calculated from the following formula (2):
                    Sa        =                              ∑                          n              =              0                        j                    ⁢                      (                                          ∑                                  m                  =                  0                                i                            ⁢                              (                                                      k                    nm                                    ⁢                                      T                    n                                    ⁢                                      Q                                          *                      m                                                                      )                                      )                                              (        2        )            
T indicates motor torque, Q* indicates a target air flow, knm (n=0, 1, 2, . . . , j, m=0, 1, 2, . . . , i) indicates a constant, i indicates a finite value, and j indicates a finite value. The formula (2) represents a relation in which the motor speed required to allow the air flow to become the target air flow Q* in a case where the motor torque is T is Sa.
The constant knm is a coefficient unique to the blower unit, and is derived by conducting the measurement experiment preliminarily. In this measurement experiment, initially, the blower unit is connected to wind tunnel experiment equipment, and the motor torque and the air flow are measured while changing the motor speed under different static pressures. Then, the value of the constant knm is calculated using a least square method so that the formula (2) becomes a regression formula which satisfies this measurement data on average.
The motor control device disclosed in Patent Literature 1 implements the constant air flow control by performing the speed control in such a manner that the command speed S* is calculated according to the formula (2) to which the constant knm derived preliminarily by the measurement experiment is applied and the formula (1), and the motor speed S tracks the command speed S*, in the manner as described above.